In the case of dissolving, in a polymer, sulfur commonly used in rubber compositions for tires (e.g. breaker toppings, base treads, clinches, sidewalls and inserts), since the sulfur has an S8 structure and also has about the same degree of polarity as ethylene glycol, it is generally difficult to uniformly disperse the sulfur in natural rubber, butadiene rubber or styrene-butadiene rubber each of which has a low polarity.
To address this problem, some approaches which reduce the amount of S8 by employing, for example, a sulfur-containing hybrid crosslinking agent (1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane) or an alkylphenol-sulfur chloride condensate have been proposed. However, sulfur-containing hybrid crosslinking agents are generally expensive, and alkylphenol-sulfur chloride condensates have a poor dispersibility themselves, thereby deteriorating the elongation at break and abrasion resistance.
A relatively large amount of sulfur is compounded in steel cord-covering rubber compositions for the sake of adhesion to cords, but because most of the sulfur is present as poorly dispersed agglomerates and does not participate in crosslinking, the crosslink density between polymers becomes non-uniform, as a result of which the elongation at break tends to decrease markedly. Accordingly, there exists a need for a compounded rubber in which sulfur is uniformly dispersed to achieve an excellent elongation at break.
Good performances such as handling stability and fuel economy are desired in all tire components. For example, the use of a tin-modified butadiene rubber having a strong bonding force to filler in order to improve fuel economy while maintaining the handling stability has been proposed.
Yet, the improvements in elongation at break and fuel economy achieved by the above techniques still leave something to be desired. Further advances in the form of significant improvements in both of these performances while maintaining the handling stability have been sought. For example, Patent Document 1 discloses improvements in performances such as grip performance using a specific styrene-butadiene rubber and coumarone-indene resin, but does not investigate improvements in fuel economy and elongation at break.
Patent Document 1: JP 2006-124601 A